1. Technical Field
The present disclosure relates to fueling a system and more specifically to a Cooperative Service Valve that provides for robotic and human compatible transfer of liquid and gaseous media, such as propellants, pressurants, coolants or life-support fluids, between assets whether on orbit or on earth and can apply to robotic or manual operation.
2. Introduction
Satellites have for many years been built with the concept that once they are brought into orbit, they would have a certain supply of fuel that, when depleted, would not be replenished and the satellite would be abandoned. Thus, a potential, and likely, end-of-life event for a satellite is the depletion of propellant. Historically, satellites which consume all of their launched propellant, but whose hardware and software components are otherwise still functioning properly, will be decommissioned or de-orbited because the lack of onboard propellant does not permit proper spacecraft attitude and navigational control. The primary means to transfer fuel into a satellite's propulsion system is through what are called Fill and Drain Valves (FDVs). What follows is a description of the various structural issues surrounding the use of FDVs which illustrate the need for an improved system for engaging with and providing media to a device like a satellite.
The primary interface for the transfer of media, such as propellants and pressurants, to a device like a satellite is the FDV. Used throughout industry, these interfaces have a number of features that can hamper robotic servicing. These features include but are not limited to the use of non-captive external closeout caps, sometimes referred to as secondary/safety caps and tertiary caps; lack of thermal isolation; lack of a controlled reaction of torque in the absence of a backing wrench; and use of lock wire to prevent backoff of closeout caps and actuation of the valve. There is also a lack of universal layout to these FDVs with respect to the spacecraft bus structure which further exacerbates access to these areas for refueling of non-cooperative satellites. In addition, there is a lack of layout and worksite standardization, and the valves themselves are also not standardized. For example, the cap size and interface, valve size, and actuation method vary between different valve manufacturers. All of these factors add complexity to the refueling (or other media transfer) task. The term refuel is used inclusively as referring to the resupply of propellant and required pressurants or other media that allow a satellite with depleted fuel stores to extend its life.
A particularly difficult disadvantage of currently available FDV's results from the fueling interface not being designed for robotic mating. The FDVs are “non-cooperative” in that they were not designed with on-orbit servicing in mind. Compared to a cooperative servicing interface, non-cooperative interfaces pose unique difficulties and challenges that need addressing. These and other challenges with respect to how one might refuel a satellite are addressed in the present disclosure.
Most FDVs feature a standardized 37 degree, flared fitting end per SAE AS4395, a design that is poorly adapted for use with a robotic system. The fitting ends are threaded which can cause risks in any threading operation such as galling between the mating threads and cross-threading. This risk increases when there is a lack of design control over both sides of the interface and the task is executed robotically and in a dynamic environment. A dynamic environment can be any environment that changes depending on where the media transfer is taking place. In the context of robotic servicing, the change comes from natural oscillations between a Servicing Vehicle and a docked Client as well as from control stability of the robotic arm. In the context of planetary robotic servicing, a change may refer to robot arm stability and atmospheric conditions like wind. In general, a dynamic environment means any relative motion between the media transfer tool and the worksite that makes it difficult for the robot operator (or autonomous control software) to position the tool where commanded.
The use of caps for seal redundancy is a disadvantage. Standard industry FDVs provide redundant means of sealing by using a flared tube cap, such as AN929, with a non-captive, non-reusable conical seal. In some cases, a tertiary cap is used to provide an additional barrier to leakage through the main valve seal or the AN cap. This standard has worked well for ground-based fueling. However, it necessitates additional specialized tools in-orbit to remove, capture, and dispose of these caps and conical seal, which introduces added mission risk and extended operational timelines. Reestablishing seal redundancy after media transfer by re-installing these caps is also extremely risky due to the high risk of galling and cross threading. On-orbit rethreading is only recommended when using a precisely controlled interface whose threads have been designed to prevent galling, eliminate the possibility of cross-threading, and/or ensure perfect alignment prior to thread engagement. As such, one approach for non-cooperative refueling has been to replace the redundant seals created by the various external caps with new controlled hardware, designed to tackle these problems, which acts as a replacement for the removed caps.
While FDVs are an interface used industry-wide for propellant systems in satellites, there is no standardization of form, fit, or function for the interface with the exception of the use of standardized flared fitting ends per SAE AS4395. FDVs cannot universally be swapped out between manufacturers or even from within the same manufacturer's catalogue due to changes in form, fit, or function. This results in the need for a refueling tool system with an adaptable front end that can accommodate a catalogue of FDVs from multiple manufacturers. For example, the use of non-standardized tertiary caps also causes problems for robotic servicing of FDVs. Due to the non-standardized design of tertiary caps, servicing of a non-cooperative satellite requires a unique tool to acquire, capture, remove, and stow the tertiary cap in order to access the FDV for refueling.
A lack of mechanical coupling is a further disadvantage. Most FDVs require thermal isolation in order to properly control the temperature of the FDV along with the propellants present in the FDV. This isolation is accomplished by preventing the mounting structure from acting as a heat sink, because current FDV designs are not inherently thermally isolated. To accomplish this isolation, thermal spacers/washers are used which also result in a poor mechanical and structural coupling between the FDV and the satellite. FDVs are usually poorly mechanically coupled to the surrounding structure, resulting from the need to thermally isolate the FDV from the surrounding structure, because a strong mechanical coupling usually results in strong thermal coupling as well. Consequently, FDVs require the use of backing wrenches during ground operations in order to properly react torques induced by technicians engaging or disengaging the FDVs' seals and caps; failure to employ a backing wrench would cause torques to be reacted into the critical weld joint between the FDV and the spacecraft propulsion system.
Lack of alignment features can create challenges during on-orbit engagement. Currently FDVs do not have intentional alignment features which would facilitate tele-operated acquisition, nor do spacecraft possess alignment aids on or within the surrounding structure. Although it is possible to use existing features and geometry, testing has indicated that dedicated alignment features aid robotic operations, promoting correct orientation of mating interfaces and thus decreasing operation timelines and the possibility of reattempts to correct for misalignments. Further, while some have displayed the ability to interface with current FDVs in-orbit, the fact remains that current FDVs and their respective satellites were never designed to be manipulated in-orbit, nor were they intended to be accessed robotically. The FDV has historically been designed around ground-based use by a human operator wearing personal protective equipment.
Currently available cooperative spacecraft refueling valves are not a direct replacement for the legacy valves. The cooperative valve designed for the Orbital Express mission, for example, is not a direct replacement for a standard FDV. That cooperative valve can be used in lieu of FDVs, but the design, consisting of two integrated valves, has a mass of 2.3 kg and an envelope of 25 cm by 7 cm by 13 cm. For comparison, one standard FDV from a well-known manufacturer has an envelope dimension of 4.75 cm by 7.67 cm and a mass of 150 g. In this example, then, the cooperatively-designed servicing valve assembly requires more than 15 times the mass allocation and a much larger foot print on the spacecraft bus, and is therefore not a direct replacement for a FDV due to changes in form and fit.
The use of a lock wire on current FDVs is also an impediment to robotic servicing. Lock wire is used as a means to prevent caps and actuation nuts from inadvertent loosening during vibration and shock loads experienced during launch. Lock wires need to be severed and manipulated on FDVs prior to establishing access to the FDV in order to transfer propellant. Addressing the lock wire requires use of a specialized tool and an extended operations timeline.
The architecture of current FDVs does not provide the ability to perform ground-based maintenance. Not all FDVs are designed to be a separable assembly, meaning that it does not have the ability to be disassembled to permit servicing or cleaning of its interior components prior to and/or after integration to the spacecraft. They become less serviceable once they are welded and integrated to the spacecraft, where desired maintenance may involve the swapping of failed components that would otherwise force an inseparable FDV to be discarded.
Aside from characteristics intrinsic to the valve itself that introduce difficulty in robotic access and manipulation, current FDV's possess a distinct lack of surface area that could be used for thermal control of the valve. Lack of surface area for active thermal control can be challenging. The majority of FDVs' exterior surfaces contain features to accommodate all of the different interfaces required for fit and function. These interfaces include thread surfaces for tertiary caps, surfaces for standard wrenches (both for actuation and for a backing wrench), holes for lock wires, external O-ring glands, as well as required means of mounting to a structure. The space available on individual FDVs varies, but an available surface area of 5 cm2 is a rough average. This results in a very small area for direct active thermal control when required. As a result, FDV's are usually indirectly thermally controlled by a combination of external, auxiliary thermal blankets and resistive heating elements applied to the surrounding spacecraft structure. These blankets pose a difficult impediment to access to the FDV as they must be robotically removed or pushed aside and restrained before attempting to acquire the valve. Like other non-cooperative features of the valve, blanket removal or restraint necessitates an ensemble of specialized robotic tools to cut, manipulate, and restrain, which introduces mission risk and impacts operational timeline.